The present invention relates to directional solidification apparatus and processes wherein heat is removed unidirectional from a melt in a mold to form a columnar grain or single casting.
In the manufacture of components, such as nickel base superalloy turbine blades and vanes, for gas turbine engines, directional solidification (DS) investment casting techniques have been employed in the past to produce columnar grain and single crystal casting microstructures having improved mechanical properties at high temperatures encountered in the turbine section of the engine.
In the manufacture of turbine blades and vanes using the well known DS casting xe2x80x9cwithdrawalxe2x80x9d technique where a melt-filled investment mold residing on a chill plate is withdrawn from a casting furnace, a stationary thermal baffle has been used proximate the bottom of the casting furnace to improve the unidirectional thermal gradient present in the molten metal or alloy as the investment mold is withdrawn from the casting furnace. The baffle reduces heat loss by radiation from the furnace and the melt-filled mold as the mold is withdrawn form the casting furnace.
In attempts to improve the thermal gradient, various baffle constructions have been proposed such as, for example, described in U.S. Pat. No. 3,714,977 where a movable upper baffle and fixed lower baffle are used and in U.S. Pat. No. 4,108,236 where a fixed baffle and a floating baffle below the fixed baffle and floating on a liquid coolant bath disposed below the furnace are used.
U.S. Pat. No. 5,429,176 discloses a cloth-like baffle that has a slit or other opening with peripheral edges that engage the melt-filled mold during withdrawal from the furnace.
U.S. Pat. No. 4,819,709 discloses first and second opposing, movable heat shields having overlapping regions that define an aperture through which the melt-filled mold is withdrawn. The heat shields are movable toward or way from one another in a horizontal plane.
It is an object of the present invention to provide multi-stage thermal baffles for DS apparatus and processes that allows tailoring and improvement of the thermal gradient in the molten metal or alloy for different mold geometries.
The present invention provides apparatus as well as method for DS casting using multi-stage thermal baffle system disposed at a lower end of a DS casting furnace. The multi-stage thermal baffle system comprises a fixed primary baffle disposed at the lower end of the casting furnace and one or more secondary baffles initially releasably disposed adjacent and below the primary baffle prior to withdrawal of the melt-filled mold from the casting furnace. The primary baffle includes a primary aperture oriented perpendicular to the mold withdrawal direction and having a cross-sectional configuration tailored to accommodate a relatively large exterior region or profile of the melt-filled mold, such as a relatively wide region of a mold corresponding to a platform region of a turbine blade or vane. Each secondary baffle includes a secondary aperture also oriented perpendicular to the mold withdrawal direction and having a cross-sectional configuration tailored to accommodate a relatively smaller exterior region or profile of the melt-filled mold, such as a narrower region of a mold corresponding to an airfoil of a turbine blade or vane.
A lower level secondary baffle remains adjacent and immediately below the primary baffle during withdrawal of the mold from the furnace until the relatively larger region of the melt-filled mold passes through the primary aperture to engage and release the secondary baffle from a temporary baffle support to allow the secondary baffle to drop or move downwardly onto the chill plate for continued movement therewith as the melt-filled mold continues to be withdrawn from the furnace.
An additional upper level thermal baffle may be used and placed above the mold and the lower level baffle. For example, the upper level baffle resides at a position above a platform region of the melt-filled mold to improve thermal gradient in the molten metal above the platform region.
Such multi-stage thermal baffle system allows tailoring and improvement of the thermal gradient in the molten metal or alloy as the mold is withdrawn form the casting furnace. In particular, the baffle apertures can be tailored to particular mold exterior profiles or configurations as necessary to improve the thermal gradient for different mold/component geometries.